A rubber boat or a boat with foam material has the advantage of lightness. Furthermore, a boat filled with foam material has high strength and high load bearing, hence they are widely applied to lifeboat, transport boast, fishing boats and so on.
A conventional foam material filled boat has a keel structure on the bottom which supports a buoyant body of the boat, thereby bending of the buoyant body is avoided. The keel structure is downward depressed, hence a convex part and a groove are formed.
One of the purposes of building a groove in the keel structure is to reduce weight. Anyway, after the keel structure is set on the bottom of the bottom plate, the bottom plate can close the groove of the keel structure to build a closed space. Thereby buoyancy of the boat is also improved.
However, since the convex part of the keel structure lies under water, it can become damaged accidently. If the keel structure becomes damaged accidently, water will seep in the keel structure, so that buoyancy of the boat is impaired.
In view of these disadvantages the inventor tried the continuous testing and improvement and developed the present invention.